1. Field of the Invention
The present invention pertains to a linear hydrophonic antenna, formed by omnnidirectional and acoustically transparent hydrophonic sensors. It also pertains to an electronic device for removing left/right ambiguity, associated with this antenna.
2. Description of the Prior Art
A linear hydrophonic antenna is generally constituted by a "string" of omnidirectional hydrophonic sensors contained in a sheath filled with an insulating oil. This sheath contains also the electronic modules associated with these sensors, this linear assembly forming a "streamer" that is generally towed by a vessel.
A linear antenna of this type forms a passive sonar system, the directivity characteristics of which do not make it possible to know whether the detected object (such as a sea vessel or underwater vessel) is to the right or to the left, above or below.
Since the distances of detection are high (generally several kilometers), this amounts to having a right/left (or starboard/portside) ambiguity which, for obvious reasons, must imperatively be removed.
Generally, several channels are formed in several directions in such a way that, for example, the observation sector is of the order of 120 degrees. The formation of these channels takes place in a very conventional way by the rephasing of the signals of the hydrophonic sensors for a given direction.
However, since the hydrophonic sensors are omnidirectional, it is not possible to have a priori knowledge of whether the signal received on a channel corresponds to a portside direction or to a starboard direction.
To remove this left/right ambiguity without any device specially designed for this purpose, the vessel may maneuver, but this very simple method constitutes an operational handicap that rules it out because of its slowness.
There is a known way to confer a certain degree of directivity on hydrophonic sensors by making them in the form of DIFAR (Direction Finding And Ranging) type modules.
Each hydrophonic module is therein formed by four sensors forming two orthogonal dipoles. A linear hydrophonic antenna of this type is described in the French patent application No. 88 16 803, filed by the Applicant on 20th December 1988. It is a linear modular antenna for which each hydrophonic module has two pairs of hydrophonic sensors forming two orthogonal dipoles, said module being associated with means for identifying the direction of the wave sources picked up. Each of these modules is then formed by a parallelepiped-shaped support bearing a hydrophonic sensor (surface sensor of pressure in this example) on each of its four lateral faces. The drawback of such a device lies in its complexity and, hence, its cost price.